Soy protein adhesive and uses thereof

ABSTRACT

The present invention provides a soy protein-based adhesive composition and a process for making the composition that may be used in particleboards and similar wood composite systems and that is formed from an aqueous mixture of calcium oxide, pine oil, protein, lignin, and acid. The preferred protein is an enzymatically-modified soy protein and the preferred enzyme is urease.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Patent Application Ser. No. 61/100,176 filed Sep. 25, 2008. The entirety of that provisional application is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to soy protein-based adhesive compositions and a process for making such compositions that may be used in making particleboard and similar wood composite systems. The invention is further directed to particleboard and similar composite systems made with such adhesive composites. Specifically, the present invention utilizes enzymatically-modified soy proteins.

BACKGROUND OF THE INVENTION

Soy and other protein-based adhesives have been used in composite wood systems prior to the advent of synthetic resins such as urea formaldehyde-based resins. The synthetic resins became popular as they offered better moisture resistance and lower press times. The demand for natural, environmentally-friendly wood composite adhesive compositions has been ever-increasing, due also in part to the known hazards of formaldehyde-based adhesives. The use of natural soy protein-based adhesives in wood composites that also resist water intrusion must result in composites that at a minimum meet or exceed the composites that utilize urea formaldehyde. The present invention that uses an enzymatically-modified soy protein satisfies these requirements.

Wood composite panels are typically formed by combining appropriate wood furnish with suitable polymeric adhesives, forming them into a mat, and finally thermally curing them under pressure. Currently the vast majority of wood adhesives employ formaldehyde-based resins. To be effective, a wood adhesive must wet wood substrates efficiently, adhere or glue particles together and, in the process, establish inter-particle bonds that create strong, water-resistant wood composites. For instance, particleboard manufacture employs urea formaldehyde (UF) resins that are typically supplied at 65 weight % solids and used at levels of 6-10 weight % of dry resin on wood furnish. However, particleboards liberate formaldehyde during production and also over their service lives. Although formaldehyde-based resins may be efficient and cost effective as adhesives, the toxicity and carcinogenicity of formaldehyde to humans is a serious health concern, according to the International Agency for Research on Cancer (IARC). Formaldehyde has a pungent, irritating odor even at very low concentrations (<1 ppm). Individuals sensitized to formaldehyde may experience headaches and minor eye and airway irritation at levels below the odor threshold. Low-dose acute exposure can result in headache, rhinitis, and dyspnea, while higher doses may cause severe mucous membrane irritation, burning, lachrymation, and lower respiratory effects such as bronchitis, pulmonary edema, or pneumonia. In May 2006, a public interest group conducting indoor air testing in Federal Emergency Management Agency (FEMA)-issued trailers in Louisiana and Mississippi reported that 94% of the trailers had indoor levels of formaldehyde in excess of that identified by the Environmental Protection Agency (EPA) and Consumer Products Safety Commission (CPSC) as triggering adverse health effects in humans. Consequently, environmental regulations governing formaldehyde emissions are slated to become more stringent in the future. For example, California's Air Resources Board required particleboard producers to limit formaldehyde emission levels to 0.18 ppm by January 2009 and requires the producers to limit such levels to 0.08 ppm by January 2011.

Other patents relate to biodegradable adhesive compositions and composites, but none employ soy protein as the sole binder in a storage-stable adhesive to yield particleboard or wood composite systems that meet or exceed ANSI specifications as does the present invention. Additionally, U.S. Pat. No. 7,081,159 is limited by solid content being restricted to <30% due to viscosity concerns. Specifically, the present invention utilizes an enzymatically-modified soy protein and preferably uses a urease enzyme for such modification. The urease modification disclosed yielded unexpected improvements in adhesive solid content and particleboard internal bond values, both of which are highly desirable attributes for such adhesive compositions. Although the use of trypsin and other enzymes have been described in enzymatically modifying proteins, there is no mention of urease being used for this purpose such that the improvements in compositions achieved with urease are indeed unexpected. Particleboards produced using the soybean protein-based adhesive produced using the present invention perform as well as or better than commercial particleboards and are an improvement over particleboards produced by the composition described in the '159 patent.

Therefore, a need exists for a natural, soy protein-based adhesive composition that can be used to make improved wood composite systems. The present invention provides such an adhesive composition which utilizes an aqueous mixture with a soy protein modified by an enzyme, specifically urease, and provides a process to make such composition and novel wood composite systems produced by such process and using the novel adhesive composition.

SUMMARY OF THE INVENTION

The present invention is directed to a soy protein-based adhesive composition, process for making the composition and a process for making the composition. In a preferred embodiment, the composition of the present invention comprises an aqueous mixture of calcium oxide, pine oil, soy protein, lignin, and acid, where the soy protein is modified by an enzyme, specifically urease. The acid can be an organic or inorganic acid. The adhesive composition can be mixed with a source of lignocellulose to make particleboard or other wood composite systems.

With the foregoing and other objects, features, and advantages of the present invention that will become apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention and to the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The adhesive composition of the present invention is an aqueous mixture of calcium oxide, pine oil, protein, lignin, and acid. The composition can be mixed with wood furnish or other source of lignocellulose to produce improved particleboard or other similar wood composite systems that are formaldehyde-free. In a preferred embodiment, the composition comprises a mixture of water, calcium oxide, pine oil, soy protein, lignin, and acid, where the soy protein is modified by an enzyme, specifically urease. The soy protein can be soybean or similar protein. The acid can be an organic or inorganic acid. Preferably, the mixture comprises inorganic nitric acid as the preferred acid.

For the present invention, the urease enzyme-modified soy protein of the mixture enables much higher solids content for the mixture and provides much improved particleboard and particleboard performance by reducing water quantity in the process of manufacture, which in turn reduces processing time and increases particleboard strength. While commercial defoamers and antimicrobials have been and can be used, the pine oil of the mixture of the present invention acts as a natural product defoamer (for improved foam reduction) and prevents microbial degradation (for improved storage stability) of the novel adhesive composition.

Currently, commercial particleboards employ urea-formaldehyde (UF) resin as the adhesive to bind wood furnish and are known to emit formaldehyde over their service lives. UF resin is supplied at 55-65% solids with a water-like consistency. Moreover, the adhesive formulation of U.S. Pat. No. 7,081,159 is limited to 30% solids with a paste-like consistency. The present invention is a vast improvement over the formaldehyde-based adhesives and over the soybean protein-based adhesive described in U.S. Pat. No. 7,081,159, which is limited to 30% solids with a paste-like consistency.

Singularly, an aqueous dispersion of soybean protein is a poor adhesive as the majority of polar and non-polar groups are rendered unavailable for wood particle wetting and adhesion. Enzymatic modification of proteins can be tailored to minimize undesired side reactions and improve access to protein functional groups. For instance, trypsin is known to hydrolyze peptide bonds containing basic amino acids such as lysine or arginine. Chymotrypsin, pepsin, and papain each selectively catalyze the hydrolysis of peptide bonds in different regions of the protein. There are currently no known reports of enzymatically modifying soybean protein to achieve improved adhesive properties for use in particleboards.

The present invention has proven significant success in enzymatically modifying commercially available soybean protein grades as the sole binder for use in adhesive formulations through selective chain scission where the adhesive processing characteristics and bond strengths are improved as molecular weight is reduced, which is an unexpected but highly desirable consequence. In a typical procedure, an aqueous dispersion of soybean protein was blended with the enzyme urease at various weight ratios for 2.5 hours at 1200 rpm (pH 8, 37° C.). The enzyme was deactivated by raising the system pH to 11, and the hydrolyzed blend was processed to an adhesive using a standard formulation. The modification enabled the synthesis of adhesives with 45% solids while achieving lower viscosity than previously achieved. A 4-liter reaction flask and high torque mixer with a 3-blade agitator was used to manufacture the adhesive. Adhesive compositions prepared from the urease-modified soy protein were processed into particleboards using a 24-inch by 24-inch press with an initial pressure of 2500-3000 psi maintained for 300-360 seconds at 193° C. The particleboards were then evaluated for density, modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond strength (IB) (Table 1).

Example Method: (Time Units are Hours:Minutes)

Time Step Detail 0:00 1 Charge reactor (reaction kettle) with water and adjust to pH 8 using ammonia. 0:05 2 Place in a water bath at 37 ± 2° C., initiate agitation at 100 RPM, and allow the temperature to stabilize for 15 minutes. 0:20 3 Add the enzyme and increase agitation to 300 RPM. 0:35 4 Allow mixture to stir for 25 minutes. 1:00 5 Add 50% of the protein over 15 minutes. 1:15 6 Add the remaining protein slowly, taking care to ensure that it is blended completely. Gradually increase stirring speed to 1200 RPM. 2:00 7 Allow the mixture to stir for an additional 90 minutes. 3:30 8 Raise the temperature to 68 ± 2° C., and charge reactor with calcium oxide and pine oil. 3:35 9 Allow the mixture to equilibrate for 30 minutes. 4:05 10 Charge the reactor with lignin. 4:10 11 Allow the mixture to stir for 15 minutes. 4:25 12 Slowly charge the reactor with nitric acid until a pH of 7 is reached. Gradually increase agitation to 1500 RPM. 4:45 13 Allow the mixture to stir for 15 minutes. 5:00 14 Transfer to storage container.

Material Weight (grams) Weight % A. Water 1504.0 50.13% B. Calcium Oxide 87.8 2.93% C. Pine Oil 47.4 1.58% D. Protein 1151.1 38.37% E. Lignin 87.1 2.90% F. Nitric Acid, 70 wt. % 122.7 4.09%

TABLE 1 Particleboard Properties MOE Density MOR (psi × IB Sample (lb/ft³) (psi) 10⁶) (psi) Protein:enzyme ANSI specification 40-50 ≧2103 ≧3.26 ≧65 (by weight) Commercial 50.50 2314 3.98 90.0 particleboard Control 50.63 1700 3.36 88.7  50:1 51.41 1991 3.39 96.6 100:1 51.27 2313 4.85 132.8 250:1 52.42 2112 4.63 151.1 500:1 52.27 2313 4.55 132.4 1000:1  51.96 2577 5.48 138.5 MOR: Modulus of Rupture MOE: Modulus of Elasticity IB: Internal Bond Strength

The data in Table 2 was obtained on an ICI cone and plate viscometer using spindle #3 at 25° C. The results show that the adhesive composition of the present invention provides higher solid content and lower viscosity than the test adhesive (from the '159 patent).

TABLE 2 Protein:enzyme weight ratio Control 1000 to 500 to 250 to 100 to Shear adhesive (25% 1 (40% 1 (40% 1 (40% 1 (40% rate solids) solids) solids) solids) solids) (1/sec) (Poise) (Poise) (Poise) (Poise) (Poise) 133 199.0 63.0 68.0 82.0 100.0 1333 18.4 8.5 8.1 9.0 10.0 13333 4.0 2.2 2.7 2.7 2.5

During particleboard manufacture, the core tightens less than either surface that is in contact with the press platens; therefore, its strength is also lower than that of the surface. The IB value of particleboards is often therefore a better indicator of particleboard performance than MOR and MOE. It has been shown that particleboards with similar MOR and MOE values displayed varying LB values that were related to differences in the manufacturing process.

The data disclosed reveal that the urease soybean protein modification of the present invention at weight ratio of 250:1 resulted in particleboards that were totally free of synthetic formaldehyde precursors and met and/or exceeded ANSI specifications for MOR and MOE, as well as outperformed commercial particleboards. More importantly, the IB values of particleboards formulated with the urease modified soybean protein adhesive of the present invention far exceeded the IB values of either commercial particleboards or ANSI specifications. The particleboards having high IB values imparted by the urease-modified soybean protein adhesive composition of the present invention are a significant improvement over current particleboards and represent a step change in particleboard technology.

The present invention has for the first time described and fully characterized an improved soy protein-based adhesive composition, process for making the composition, and wood composite system made with the adhesive composition. The above detailed description is presented to enable any person skilled in the art to make and use the invention. Specific details have been disclosed to provide a comprehensive understanding of the present invention and are used for explanation of the information provided. Descriptions of specific applications are meant to serve only as representative examples. Various suitable changes, modifications, combinations, and equivalents to the preferred embodiments may be readily apparent to one skilled in the art and the general principles defined herein may be applicable to other embodiments and applications while still remaining within the spirit and scope of the invention. The claims and specification should not be construed to unduly narrow the complete scope of protection to which the present invention is entitled. It will be understood by those skilled in the art that the present invention is not limited in its application to the details of the arrangements described herein since it is capable of other embodiments and modifications. Moreover, the terminology used herein is for the purpose of such description and not of limitation. Therefore, the invention is to be accorded the widest possible scope consistent with the principles and features disclosed herein and the description herein is not to be considered limiting in scope. 

1. An adhesive composition comprising a mixture of: water; calcium oxide; pine oil; enzyme-modified soy protein; lignin; and an acid.
 2. An adhesive composition comprising an aqueous mixture of: a hydrolyzation agent; a defoamer; a source of enzyme-modified soy protein; a source of lignin; and an antimicrobial agent.
 3. The adhesive composition of claim 1, wherein the soy protein modifying enzyme is urease.
 4. The adhesive composition of claim 3, wherein the acid is an organic acid.
 5. The adhesive composition of claim 3, wherein the acid is an inorganic acid.
 6. The adhesive composition of claim 5, wherein the inorganic acid is nitric acid.
 7. The adhesive composition of claim 6, comprising about 50% water; about 3% calcium oxide; about 2% pine oil; about 38% enzyme-modified soy protein; about 3% lignin; and about 4% nitric acid.
 8. A particleboard comprising: a binder comprising a mixture of water, calcium oxide, pine oil, enzyme-modified soy protein, lignin, and an acid; and a source of lignocellulose.
 9. The particleboard of claim 8, wherein the soy protein modifying enzyme is urease.
 10. The particleboard of claim 9, wherein the acid is an organic acid.
 11. The particleboard of claim 9, wherein the acid is an inorganic acid.
 12. The particleboard of claim 11, wherein the inorganic acid is nitric acid.
 13. The particleboard of claim 8, wherein the source of lignocellulose is wood furnish.
 14. The particleboard of claim 12, wherein the binder is comprised of about 50% water; about 3% calcium oxide; about 2% pine oil; about 38% enzyme-modified soy protein; about 3% lignin; and about 4% nitric acid.
 15. A process for preparing an enzyme-modified soy protein adhesive composition, the process comprising the steps of: loading an effective amount of water in a reaction kettle; adjusting the pH and temperature of the water for optimal enzymatic activity; adding at least one enzyme to the water to form a mixture and allowing the mixture to stabilize; adding at least one soy protein sequentially to the mixture under constant agitation; continuing agitation for an additional 2-6 hours; neutralizing the enzyme activity by adjusting reaction conditions; adding effective amounts of calcium oxide, pine oil, lignin, and acid to the mixture; and adjusting the pH of the mixture to about 7 before unloading the mixture.
 16. The process of claim 15, wherein the at least one enzyme is urease.
 17. The process of claim 16, wherein the acid is an organic acid.
 18. The process of claim 16, wherein the acid is an inorganic acid.
 19. The process of claim 18, wherein the inorganic acid is nitric acid.
 20. A soy protein adhesive composition made by the process of claim
 15. 